This invention relates to a slide fastener which includes a continuous fastener element row secured to one side edge of a fastener tape by means of weaving, knitting or sewing. The fastener element row is produced by extruding synthetic resin material into a series of connected molding cavities in the periphery of a rotary die wheel and then drawing the molded product out of the die wheel.
A synthetic resin fastener element row having a plurality of fastener elements in series connected by spacer means produced by means of an extruding means is useful because the individual fastener elements are regularly spaced from each other and the fastener element row having a long length can be obtained. For producing such a fastener element row, it has been proposed to extrude synthetic resin material in the form of a sheet by a die and then subject the molded product to a processing step such as punching or bending to thereby obtain a fastener element row including a plurality of individual fastener elements having engaging heads and upper and lower legs the free ends of which are connected together by portions of synthetic resin material. However, such a fastener element row has portions connecting between the individual fastener elements and having the thickness of the material sheet and an increased width because the fastener element row is produced by punching the material sheet and the fastener element row lacks in flexibility required for the same. Thus, the fastener element row is unsuitable as a product.
Therefore, study has been conducted with the aim to produce flexible fastener element rows and various proposals have been made. According to the art disclosed in U.S. Pat. No. 3,328,857, for example, the upper and lower legs of each individual fastener element are connected to the corresponding legs of adjacent fastener elements by means of a cord and the cords between the successive adjacent legs are alternately covered with synthetic resin to connect the legs together. Such synthetic resin coatings on the upper and lower legs of each fastener element are staggered. In the slide fastener of this type, since alternate cords are left uncoated and exposed, the exposed cords exhibit no load supporting function and thus, when high lateral pulling or bending force or thrust is applied to the slide fastener, separation extends from the bare cords through the legs to the coated cords and impairs the function of the slide fastener. In an extreme case, such separation damages the slide fastener. When the fastener element row is secured to the fastener tape, the element row can be secured to the tape by weaving; in such a case, threads constituting the tape are placed about the coated and uncoated portions. However, since the coated and uncoated portions have different load bearing capabilities, the individual fastener elements tend to incline to make it difficult to secure the fastener element to the tape with constant pitch.
In order to eliminate the above-mentioned disadvantages, the applicant proposed the slide fastener as disclosed in Japanese Patent Applications Nos. 188386/1980 and 186005/1980. In the slide fastener disclosed in these Japanese patent applications, synthetic resin spacer means are provided between adjacent fastener elements for resiliently bending together with the fastener elements and the spacer means are positioned in the position corresponding to the vertical axis of the fastener element row passing through the center of rotation of each of the fastener elements about which the element rotates as the slide fastener element row is opened and closed to thereby eliminate the above-mentioned disadvantages. More particularly, the slide fastener of these Japanese patent applications exhibits stabilized function with proper and smooth sliding resistance of the slider as the slide fastener is opened and closed.
As compared with the prior art slide fasteners, although the slide fastener of these Japanese patent applications is substantially improved with respect to thrust (vertical force acting concentrically on the element engaging portions of the slide fastener) strength and bending (force for bending the slide fastener in the longitudinal direction of the slide fastener) strength, the slide fastener is not satisfactory for use in connection with bags and trousers where substantial thrust and bending force act. In order to further improve the thrust and bending strength of a slide fastener, although it is known that the spacer means are preferably positioned adjacent to the free ends of the legs, (When thrust or bending force is applied to the fastener, the fastener elements positioned in the position where the peak of the thrust or bending deformation acts tend to widen the distance therebetween and at the same time, the engaging heads of such elements tend to rise up pushing the engaging heads of the adjacent elements away therefrom. At this time, if the distance from the engaging heads to the spacer means is long, the adjacent elements can move by a great distance maintaining their engaging relationship. And when the distance from the engaging heads to the spacer means is long, the elements easily deform elastically and resist disengagement.) When the spacer means are positioned adjacent to the free ends of the element legs as stated above, the spacer means are displaced from the neutral axis of the fastener element row and as a result, the degree of expansion and contraction of the spacer means increases as the fastener is opened and closed and the sliding resistance of the slider increases. And, as the fastener is frequently opened and closed, when the expansion and contraction of the spacer means increase as mentioned above, the spacer means become fatigued which causes the boundary between the spacer means and legs of the fastener element row to crack to thereby shorten the service life of the fastener.